Ground controlled approach indicator system



Aug. 13, 1963 A. BROCKWAY, JR 3,100,891 GROUND CONTROLLED APPROACH INDICATOR SYSTEM Original Filed 001;. 26, 1954 15 Sheets-Sheet 1 (FIG. I

ARTHUR L. BROCKWAY, JR.

IN V EN TOR.

ATTORNEYS A. L. BROCKWAY, JR 3,100,891 GROUND CONTROLLED APPROACH INDICATOR SYSTEM 15 Sheets-Sheet 5 Aug. 13, 1963 Original Filed Oct. 26 1954 I. l s fi 35:5 22:: 5 2 m n lllr v N m lllll I ma m m l $250 2: :55 us em 2 .28 ram $55 2.5; E d v B $252 52% 59 32 3? Jo a? a; 5 mi 55E m g LEEVEE $2550 JIQH uz E355: ICU-W 8 m a u A 53 was; =2: 3:2 Q 2:2 2 58.5 50 N 6 535-5 LULJ. 52$: E252 2 n $12? 55 m C n .E

Aug. 13, 1963 A. L.-BROCKVIVAY, JR 3,100,891

GROUND CONTROLLED APPROACH INDICATOR SYSTEM ori inal-Filed Oct. 26, 1954 15 Sheets-Sheet 5 =5 ES JE 5:25 2:0 22 5% 35 E; u E N 2: 523%: z a s IN V EN TOR.

A T TORNE Y5 Aug. 13, 1963 A. L. BROCKWAY, JR 3,100,891

GROUND CONTROLLED APPROACH INDICATOR SYSTEM Original Filed Oct. 26, 1954 1s Sheets-Sheet e INVENTOR ATTORNEYS Aug. 13,1963 I A. L. BROCKWAY, JR I 1 I GROUND CONTROLLEb APPROACH INDICATOR SYSTEM AT TORNE Y5 Aug. 13, 1963 i A. L. BROCKWAY, JR 13,

GROUND CONTROLLED APPROACH INDICATOR SYSTEM- Original Filed Oct. 26. 1954 15 sheets-sheet a:

0 VOLTS 0 VOLTS TIME ' ARTHUR L. BROCKWAY, JR.

IN V EN TOR.

FIG. 4

ATTORNEYS I V Aug. 13, 1963 A. L. 'BROCKWAY, JR I 3,

GROUND CONTROLLED APPROACH INDICATOR SYSTEM Original Filed 001;. 26, 1954 l5 Sheets-Sheet 9 i i i i iJ-HHH T iqmnn v +nnn 1- 1 MECLT'OFHELEEL' ARTHUR L. BROCKWAY, JR-

Fl 6 A JNVENTOR.

BY I

ATTORNEYS Aug. 13, 1963 A. L. BROCKWAY,JR 9

I GROUND CONTROLLED APPROACH INDICATOR SYSTEM 7 Original Filed Oct. 26, 1954 13 Shee ts-Sheet 1o DEFLECTION SYSTEM +400 as I M u I VERTICAL SPACING H0 vl I29 L e R FIG. 9A

Fac.9

ARTHUR L. BROCK-WAY,;IJR.

INVENTOR.

I-DEGREE5 0F POWN TIOI IETER ROTATION w ATTORNEYS Aug. 13, 1963 I A. L. BROCKWAY, JR 3,100,891 GROUND CONTROLLED APPROACH INDICATOR SYSTEM -15 Sheets-Sheet 11 Original Fi led Oct. 26, 1954 IN VEN TOR.

ARTHUR L.BROCKWAY, JR.

I E L 53; 5-

ATTORNEYS Aug. 13, 1963 A. L. BROCKWAY, JR

GROUND CONTROLLED APPROACH INDICATOR SYSTEM 15 Sheets-Sheet 12 Original Filed Oct. 26. 1954 IN VEN TOR.

ARTHUR L. BROCKWAY, JR.

ATTORNEYS Aug. 13, 1963 A. 1... BROCKWAY, JR

GROUND CONTROLLED APPROACH INDICATOR SYSTEM Original Filed Oct. 26, 1954 13 Sheets-Sheet 13 f; 3 -w--M f ARTHUR L BROCKWAY, JR

INVENTOR.

A TTORNE YS United States Patent 9 7 Claims. (Cl. 34311) This application is a division of Serial No. 464,825, now U.S. Patent No. 3,014,213. i

This invention relates to ground controlled approach (GCA)systems and more particularly to a system for presenting azimuth and elevation information as a composite display.

In the earliest GCA systems azimuth information was presented on one cathode ray tube and elevation information on another, each presentation being watched by a separate operator. In later systems it was realized that a reduction in the number of observers could be obtained by presenting both displays ona single tube screen. This manner of presentation has been called Az-El presentation.

In GCA installations using the Az-El form of presentation it has been customary to generatethe complete azimuth and elevation presentations in alternation. A display generated in this manner has the disadvantage of appearing to flicker since it is not possible to generate complete sectorial P.P.l. type indications rapidly enough so that persistence of vision will provide the appearance of a steady picture.

It is, accordingly, an object of the invention to provide means for generatingan Az-El presentation in line by line alternation, whereby flicker is eliminated.

Each of the Az and El presentations is a distorted sector of a P.P.I. presentation oil-centered to a difierent lo I This means that the point of origin for the sweeps has to be shifited with each line.

cation on the tube face.

An oft-centering circuit alternating between two stable conditions is required for thispurpose. One of the diffi culties previously encountered in AzEl displays has been that of obtaining a restoration of the value of current flowing in the deflection coil at the beginning of each sweep within the restoration time available. Failure to achieve such restoration results in the existence of a transient current which produces an effect known as .walking of the display, characterized by movement of theorigins.

It is another object of the invention to provide displays which may be readilyadjusted to have point. origins which are stableunder ordinary scanning conditions.

GCA presentations traditionally include. what are known as cursor and wave-off, lines. Cursor lines mark the path down which it is desired that the plane land. In the azimuth presentation the cursor line represents the center of the runway. In the elevation presentation it represents the glide path as seen from a lateral position. Wave-off lines are lines appearing on the display on both sides of and close to the cursor lines. They mark .the limits of permissible deviation of the location ota landing aircraft from the glide path. When the indication of the target aircraft appears on the display outside of the wave-oil lines of either the Az or the El presentation the GCA operator instructs the aircraft to turn away, i gain altitude and repeat the approach. This operation is termed a wave-oii'.

. In the early GCA systems these lines were physically inscribed or printed on masks which were placed over the tube face. This expedient, however, is unsatisfactory since the presentation may not always be readily posiofi lines by intensifying for each line a spot on each sweep V trace of the cathode ray beam. All the intensified spots for each line would, in the aggregate, trace the desired line. i 4

Attempts have been made to generate lines in this fias-hion, but these attempts have resulted in systems which present several defects or limitations. I

In the use of GCA systems the equipment is usually truck mounted so that it can be shifited from runway to runway as conditions demand. It is also desirable for maximum accuracy that the system be located quite close to the runway in use. In shifting the equipment from runway to runway the speed with which the operation can be accomplished is a matter of great importance.

.In attempts previously made to generate cursor and wave oli lines electronically great difficulty has been encountered in meeting the above requirements. The systems devised have necessitated the following of an elabo rate procedure of adjustment of the lines when moving from one location to a new location. This procedure involved precision setting of a number of dials, use of several correction charts and the repeating of a number of adjustment steps due to control interaction. These systems were also very sensitive to the accuracy of positioning of the equipment in accordance with a predetermined orientation. i The range of operation of these systemsis limited by the circuitry involved, rendering a close positioning of the system to. the runway. difficult to achieve.

It is a further object of the present invention to pro vide an indicator system with electronically generated cursor and wave-oil lines, in which adjustment of cursor lines in moving to a new runway location is simply and easily accomplished. I

It is another object of the invention to provide such a It is another object of the invention to provide such a system which need not be accurately positioned with respect to a runway in order to be easily adjusted for operation.

' These and other objects and advantages of the invenw tron are realized by an indicating system in which the azimuth and elevation presentations are switched after each pulse interval. generated by circuits in which a fixed amplitude sawtooth voltage is initiated during each interval between trans mitted pulses. 'llhis voltage is applied to a pick-on diode biased by a DC. voltage related to the antenna scan. po sition. The output of the pick-oil diode isapplied to wave shaping circuits which increase the rise time thereof and differentiate the resulting waveform. The positive the waveform applied to the video ClICLlliI'SQEiIld tensifioation of the sweep trace. The wave-off lines are generated by similar means.

which produces an in The dilterence in output necessary to trace the ditterent I lines arise in differences in the configurations and driving The cursor and wave-ofi lines are pulse which is- This situation remains effective as the sweep varies through the scanning range.

the azimuth and elevation systems.

linkages of potentiometers driven in accordance with the scan of the azimuth and elevation antennas.

Walking of the point of original of the Az and El displays is eliminated by means of a resistor, an inductor and a DC. voltage source serially connected and shunted across thevertical deflection yoke with the voltage so polarized and the time constants of the shunt so remb lated that the changing current through the deflection yokedue to its recovery from the preceding sweep exactly equals and opposes the current flowing through the yoke due to the recovery'of the coil from the same sweep, at the instant at which the next succeeding sweep is initiated. amplitude In the drawings:

. FIG. 1 is a plan view of a caode ray tube screen having formed on it an Az El presentation of the type contemplated by the invention; 7

FIG. 2 is a block schematic'diagram of a GCA indicator system embodying the invention;

FIGS. 3, 3A, 3B, 3C and 3D are block schematic diagrams of portions of the system of FIG. 2 showing the system broken down more specifically into its elements;

FIGS. 4 and 4A are groups of curves drawn to a common time base showing waveforms occurring at different points in the system of FIGS. 2 and 3;

FIG. 5 is a schematic diagram of a portion of the circuit of FIG. 6; FIG. 6' is a schematic circuit diagram of the deflection system of thejoverall system of FIGS. 2. and 3;

'- FIGS. 7 and 7Aare schematic circuit diagrams of the cursor-and wave-off line generating portions ofthe system S of FIGS. 2 and 3;

FIG. 8 is a diagram showing the trigonometric relationships existing between a GCA system positioned close to a runway, the center' line of the runway and the azimuth antenna scanning pattern; i

FIG. 9 is a graph of the resistance curve of one of the potentiometer-s employed in the system of FIG. 2 between the antennas and the cursor and Wave-off line generators;

FIG. 9a is a schematic diagram of a potentiometer of the type to which FIG. 9 pertain-s; and,

, FIG. 10 is a schematic diagram of a portion of the v 1 vertical sweep circuit.

The IndicatoriS'ystem Generally The Az El indicator provides in a compositeview on a single cathode ray tube (CRT) a continuous unflickering display of all of the indices in azimuth and elevation required-for the safe and eflicient landing of aircraft. The system Whichprovides these indices is arranged to perinit rapid and accurate adjustments without interaction I amongthe various circuits and a high degree of flexibility forjemployment with various runway situations. j The indicator includes verticaland horizontal deflection circuits which produce an AzEl display correspond vides switching voltages to a vertical off-centering and ing circuit I.

H is also applied tov the cursor circuits B. 1 The output ing to the respective scans of the azimuth and elevation antennas with successive range sweeps alternating between Other circuits pro vide electronic cursors on the displays which represent the azimuth runway courseline and the elevation glidepathpand waveoff lines spaced a predetermined distance "therefrom. Blanking and clipping circuits are incorporated to present the A;El display without'interference between the two portions and for maximum ultizai .tion of thescreen area of the CRT. Conventional logarithmic range marks are generated electronically and provided on the display and gating' and switching voltages various functions. v v v v Suoh'an indicatorscr'een' is shown in FIG. 1, which i1- lustr-atesa round. screen. 36 with'an azimuth (Az display 37 and an elevation (El) display 38.

the waveforms .of FIGS. 4 and 4A, the igenerationo-f' .areprovid ed to all units tocorrelate the timing ofthe On the Az dis- 5 play the cursor line is the horizontal line 39 and the two wave-oil lines are the lines 40 and 41. On the El display, the cursor line is the line 42 and the two wave-off lines are the lines 43 and 44. The line 43- is termed the No. 1 wave-oft line on the El display and represents a line in space parallel to the glide path and about fifty feet above it. The line 44, known as the No. 2 wave-off line, makes an angle with the glide path which is adjustable and is usually adjusted to a slope of about fifty feet per mile; On the azimuth display line 40 is the No. 1 waveoff line and 41 is the No. 2 line if the system is positioned to the left of the runway. Each wave-ofl? line represents a line in space making an angle with/the approach path having a slope of about two .hundred feet per mile. The wave-off lines extend for about three miles. whereas the cursor line extends for about ten miles. Range marks 45 at one mile intervals are shown. The range mark spacing is logarithmic in order that deviations from the. ap-

proach path may be more apparent as an aircraft ap The left hand range mark apdrive a group of angle voltage and cursor potentiometers A. The cursor and wave-off line voltages generated therein are applied to a group of cursor and wave-off circuits B the output of which is applied to video amplifier circuits E.

A group ofgating circuits C, driven by Az' and El pretrigger voltages generated elsewhere in the system, provide gates which are applied to the cursor circuits B and to range mark generating circuits D. The output of'the latter is applied to the video amplifier circuits E. The video Az and El signals are also applied to these circuits. The output of the video amplifiers is applied to the .control grid ofaCRT L.

The CRT is provided with an electromagnetic deflection yoke comprising vertical and horizontal deflection coils for producingthe sweeps of the cathode ray beam and these coils are driven by horizontal and vertical sweep I circuits F and G respectively. A power supply circuit K drive an Az and El switch H, the vertical sweep G, and

the blanking circuits I. The Az and El switch H pro separation circuit I and an Az and blanking andshap- Angle voltage from the angle voltage potentiometers of block A is applied to. the vertical sweeps G and the blanking circuit 1. The output of theAz and El switch of the Azand El blanking and shaping circuits I is ap plied to the cathode iof the CRT, together with the output of an hnblanking circuit N which is driven by asignal coming from the horizontal sweep circuits.

. Genera t ing the AzEl Map l Referring to the block diagram of FIGS. 3 to 3D and the AzEl map will be described. Refer-ring first to FIG. 3, Az andEl pro-triggers 1 2 which are generated elsewhere and "occur 20.8,! seconds before the respective I A2 and El transmitted pulses, waveforms 3 and 4,- are' applied to amixer 51 which triggers a monostable multivibrator 52 with an adjustable on time to produce themaster gate which is'inverted in inverter 6s to produce gate.5. Gate Sis applied to a trapezoid; generator 53 which provides an adjustable'delay in initiating multivibrator .54 to produce sweep 'gate 6. The trailing edge of .the negative going excursion of gate Sis I effective to terminate the positive going excursion of gate 6 simultaneously. The pretriggers 1, 2, also operate a bistable .m ultivibrator 55 through inverter amplifiers tubes.

respective Az and El vertical clamped trapezoid genera-.

tors 58, 59 which will be more fully described. The generators 58, 59 produce variable amplitude and polarity trapezoids 22, 23 in accordance with the respective antenna angle voltages. The trapezoids 22, 23 are mixed and amplified in a vertical sweep feedback amplifier 61, the output of which is applied to a vertical sweep driver 62. The junction therebetween is clamped to the bias voltage for the driver 62 by a clamp gate generator 63 which is switched by wave 18 from a horizontal sweep gate amplifier 60. Current [from the sweep driver 62 is drawnthrough the vertical deflection yoke system to produce the desired vertical component of beam deflection. A vertical oil-centering tube 64 alternately switches a constant value of current on and off through the vertical deflection system for providing the displacement between origins of the Az and El displays.

Referring to FIG. 3B, the horizontal deflection in the indicator of the present invention has a constant maximum value and is derived from the wave 13 which is the inverted iforrnof wave 6 applied to amplifier 60. Wave 18 switches a clamped trapezoid generator 65 which is referred to an adjustable DC. voltage for horizontal expansion control. The horizontal trapezoid 21 has a logarithmic rise and is amplified in a feedback amplifier 66, the output of which is clamped and connected toa horizontal driver 67. The driver 67 con trols the sweep current through the horizontal deflection system. i

The basic difference between the horizontal sweep Waveform. 21 and the vertical sweep waveforms 22 and 23, as seenin FIG. 4A, is that the vertical sweep waveforms vary in both amplitude and polarity whereas the horizontal sweep waveform varies only in amplitude. Furthermore, once the amplitude of the horizontal sweep has been adjusted to give the desired presentation, it is not altered.

In the functioning of the vertical sweep circuit, as illus trated in partin FIG. 10, during the time of the azimuth sweep the negative 122 microsecond gate output 7 of the Az sweep gate coincidence tube 56 is applied tothe control grids of tubes 49a and 49b. The cathode of tube 49a and plate of tube 4% are connected by way of load resistors to ground. The cathode of 4% is con nected to the negative terminal of the 150 volt source and the plate of 49a is connected tota source of positive voltage such that the outputs of the two tubes are equal for. the same input. The output of cathodetollower 49a is applied in parallel to the plates of two diodes 76a and 77a. The output of tube 4% is applied in parallel to the cathodes of two diodes 76b and 771).

The remaining electrodes of diodes76a and 76b are grounded while those of diodes 77a and 775 are connected to a common point 50. Tubes 4% and 49b,

together with tubes 76a, 76b, 77a and 77b, constitute a push-pull clamper circuit, which will clamp to ground both positive and negative waveforms. When 4% is cutoff, a negative pulse 122 microseconds long is applied to the plates of clampers 76a and 77a cutting ofl" these At the same time, a positive 122 microsecond pulse is applied by 4911 to the cathodes of clampers 76b and 77b, cutting .off these tubes.

At the instant the clampers are cut oflthe junction 5!) of the cathode of 77a and the plate of 7712 is at ground potential. This junction is connected to one terminal of a condenser 78, the other terminal of which is grounded through a resistor 79 which is bypassed by a variable angle voltage input.

6 condenser 80. 'I'l1e point 50 is also connected by way of the arm of a potentiometer 98 to the cathode or. a triode 99 connected as a cathode follower. The point 50 is also connected to the control grid or a triode 100. Azimuth antenna angle voltage is applied to the control gridof cathode follower 99 from terminal 176.

At the instant the clampers are cut bit the point 50 is at ground potential and the voltage across condenser 78 is zero volts. If the azimuth antenna is at the center of tits travel, which is the zero-degree scan point, the Az angle volts input is Zero volts and so is the output of potentiometer 98 which is connected to the cathode of cathode follow-er 99. Thus the voltage across condenser 78 remains at zero volts.

It the azimuth antenna is at the ten degree right scan point, the voltage output of the cathode follower 99 will be some positive value. At the instant the gates are applied to cut oh? tubes 77:: and 77b there will be no, change on. condenser 78. 5 However, a jump voltage will be developed across resistor 79 and capacitor 78 will begin to charge from the jump voltage to some positive voltage, depending on. the amplitude of the angle voltage input. This will result, in the application of a positive trapezoidal voltage to the gridof tube 160.

When the angle voltage input becomes negative, the circuit functions in exactly the same manner. However, under these conditions a negative trapezoidal waveform is applied to the control grid of tube 100. The purpose 7 offlthe potentiometer 149 seen in the cathode lead of the tube 100 isto vary the :gain of the tube by inserting more or less unbypassed resistance into the cathode circuitj j j The Elvertica'l' sweep circuits are constructed and function in exactly the same manner as the Az vertical sweep circuits described above, except that the trapezoidal waveformgenerated in. the El vertical circuits goes approximately six times more negative than it does positive. The reason. is that the El antenna scans from approximately plus six degrees to minus one degree whereas the Az antenna scans from ten degrees left to th-eten degrees right. V

Continuing with reference to FIG. 10, during the time 'of the El sweep, the negative 122. microsecond pulse output 8 of the El sweep gate coincidence tube 57 is applied to the control grids of twotubes 206a and 206b, cutting oif these stages. These tubes are connected in the same manner and perform the same functions as tubes 4% and 4% described above. Their outputs are applied to a two way clamper composed of tubes Ziila, 297 b, 208a and 20817 which correspond in arrangement and function to tubes 76a and b and 77a and .b in'the Az vertical sweep circuit. As the tubes 206a and 20612 are out off the voltage at the junction point 2ii9 of the cathode "of 208 1 and the plate of2il8b will be zero volts.

This junction is connected directly tothe control grid of a tube Z-lthconnected in the same manner as tube 100 and forming a mixer-with that tube. It is also directly connectedito one terminal of a condenser211, the remaining terminal of which is connected through a resistor 212 to ground. This latter resistor is bypassed by a variable condenser 213. El antenna angle voltage is applied from a terminal 214 to the control grid of a cathodefollower Z15fWldCh serves the same function as the cathode follower 99. A potentiometer 216, is connected in its cathode' circuit and the; arm thereof is connected through a resistance to the junction point' 209.

When the tubes 206a and b arecut oif, the junction point 209 will remain at zero potential during the sweep, if the angle voltage being applied to tube 215 .is zero. If the angle voltage is other than zero volts a jump voltage is developed across resistor 212 and condenser 211 starts to charge in a direction determined by the polarity of the Condenser 213 is used. tomake the rise time of the elevation vertical sweep trapezoid the same as the rise time of the horizontal sweep trapezoid.

ployed heretofore.

Acccmplishing this tends to eliminate any hooks or curves in the start of the sweep. A potentiometer 217 in the cathode circuit of the tube 210 serves the samepurpose provide the Az and El sectors of a composite display and their operation thereof for this purpose will presently be described. In order to properly present such a display, however, certain additional circuits are required for modifying the indications and further enhancing the utiity of the system. Certain of these circuits perform the conventional function of properly clipping and shaping the composite Az-El display. Other circuits are provided which in accordance with the present invention produce an improved operational system having the aforementioned advantages over the systems that have been em-.

The Blanking Circuits In order to make the desired, portions of the display visible and blank the unwanted or overlapping portions thereof, blanking circuits are provided to control the cathode potential of the cathode-ray tube with appropriate values in the desired sequence. Referring to FIGS. 3C and 3D, the Az and El coincidence sweep-gates 7, 8 are applied to a flip-flop 71 which generates the Az-El switch waveforms 9 and 10. The wave .10 is applied by way of a switch 72 (FIG. 3D) to a cathode follower 203 which alternately passes and blocks a signal corresponding to the vertical deflection current 29,-thus producing the output waveform 30. The rise of the sweep signal trapezoid of waveform 3 is adjustably selected by a clipping .leveldiode 73 to provide an output waveform 31 which 7 is applied to a two stage amplifier .204 followed by a cathode follower with a feedback connection to the amplifier. The output of this circuit is used to trigger an Az blanking one-shot multivibrator 74, thereby generating gate 32 for blanking the Az display along a horizontal line corresponding to a fixed amplitude of vertical deflection current. This waveform-is applied to an Az blanking cathode follower 211.v p

All of the blanking of the El display occurswhen the elevation antenna is scanning the sector of zero degrees to minus one degree andnormally no blanking occurs until the sweep range is slightly in excess of one mile. During the time of the El display a negative gate (waveform 3) is applied from the El sweep gate coincidence tube. 57 (FIG. SC) to the input of a clipping sawtooth generator 205 (FIG. 3D), cutting it off. A cathode follower 206 has the elevation angle voltage applied to its grid. Its cathodeiscoupled to the plate of thesawtooth generator 205 through a double clamp 75 and conductor 207; This clamping action is effective when the generator 205 is conducting, thus tying the level of the base line of its output waveform to t-he angle voltage. The double 7 clamp '75, by a. second conductor $208 connectedrto the elevation angle voltage input, also aotsto limit the sawtooth generated by the sawtooth generator during its outoff periods to'a level slightly more negative than'the elevation angle voltage. The output of this generator is illustratedas waveform 26 ('FIGS. 3D and 4A), which is below the zero voltage line. This waveform is applied to a two-stage overdriven amplifier 209 (FIG. 3D) and cuts off the secondstage of this amplifier for all eleva-' tion angle voltage values which occur below zero degrees of elevation movement. Since the baseline of this sawtooth rises with a positively increasing elevation angle voltage, and since this angle voltage increases in a posiand 3D), is applied to an El blanking cathode follower 210, and thence to the point 82.

Intensificdtion Gate-Shading and Mixin The horizontal sweep gate 6 is inverted in amplifier 60 (FIG. SE) to form intensification wave 1 8 (FIG. 4A).

A portion of wave 6 is combined (FIG. 3D) through-a time constant circuit 81 and sweep shading cathode follower 69 with wave :18 which is passed through unblanking cathode follower 70, to form wave '25. This Wave has negative beam intensifying portions having a curved positive going shape which tends to decrease the intensity of the electron beam as the range sweep progresses. Since the range sweeps are exponential, the exponentially varying value of beam intensity produces a substantially constant intensity-velocity product for all portions of the sweep, thereby providing a uniformly visible intensity display. The duration of the intensifying gates 25 is moditied, for portions of the antenna scans where part of the display is to be blanked. Thus, during intervals when El blanking gates 27 are generated, the intensification wave 25' is modified to appear as wave 28'avith the El intensification gates variably terminated to obtain the appropriate blanking. Likewise the Az blanking gates 32 variably terminate the Az intensification gates to produce wave 33. The Az and El blankingcircuits 210 and. 211 supply a common. mixing circuit 82 whereby the Various potentials are combined for application to'the cathode 83 (FIG. 3). The potentials thus-applied are waves 25, 28 or 33 (FIG. 4A) depending upon whether the antennas are scanning in a direction for which there is no blanking, only El blanking, or only Az blanking; If the Az and El antennas are not synchronized to scan up and down together on the displays a conditionmay exist where both Az and .El blanking are present for adjacent sweeps. I 1

f The Deflection Systems The deflection, systems of the present invention are of Q the electromagnetic type arranged to provide improved performance contributing [to the accuracy and general utihty of the Az-El display. For simplicity a fundamental circuit of the type employed is showniin FIG. 5 wherer in an air cored deflection coil 91 with shunt damping res1stors90 are connected in the plate circuitofa singleended deflection amplifier 92 and to the positive termi ml of a D.'C. source 93. Connected in shunt with the deflection coil 91 and'resistors 90 is the serial combinatron of an adjustable D.C. source 94, an air cored inductor 95 and an adjustable resistor 96. Tube 92 draws a quiescent current from supply 93 through the coil 91 and variations. of the tube current in response to opposite polarity signals produce oppositely directed deflection forces. "The-rest position of'the electron beam is adj-11stable by varying the voltage of source 94. The current thru coil 91 fromsource 94 is in opposition to the coil current in tube 92 and the resultant deflection force positions theelectron beam in :a-ccordancewith' the resultant cur-rent therein. I g V i In order toposition the electron beam 'for alte'rnate Az I and El sweeps there is provided a vertical spacing tube J which switches between two fixed states, one-conductive and. the other nonconductive, at the end of each vertical sweep gate. The current drawn by the tube 64 traverses the coil 91 producing a component therein corresponding to wave 24. 1 Tube 64is provided with dea generative cathode resistors 93 returned to a negative 9. supply and with a relatively low screen voltage to ensure that the damping of coil 91 thereby for the two states remains essentially contant. The change in current wave 24 moves the rest position of the beam into the vicinity of the respective origins for the Az and El sweeps at the end of the preceding sweep and hence provides the maximum period for restoration of quiescent conditions before the start of the next sweep.

As hereinbefore explained, complete yoke recovery thru a normal resistive damping circuit is not realizable in any practicable Az-El display with shout restoration periods. Furthermore the value of current in the deflection coil will be different as the antenna angle voltage varies and thus the value of current at the end of the restoration period will vary with the angle voltage. This combination of a transient current existing in the deflection coil at the beginning of each sweep which has a value dependent upon the antenna angle voltage produces a phenomenon known as walking. This phenomenon is characterized by the appearance of the orgins moving up and down with the scanning motion of the respective antennas. Such a motion of the display origins, in addition to producing an influence tending to distract the operator, introduces inaccuracies in the information display near the origin where they can least be tolerated. The circuit of the be readilyadjusted to have point origins which are not a function of vertical centering or expansion and are stable in the presence of variations normally encountered in the supply circuits and the like. For this purpose the coil 95 and resistor 96 are provided in the shunt circuit of the deflection coil 91 with preferably a relatively large im pedance to avoid the necessity of any appreciable increase in current carrying requirements for the deflection amplifier 92. Thetime constant of the circuit including the coil 95 and resistor. 96 is made approximately equal to the time constant of the deflection coil 91. An air core choke is used for this application in order to maintain constantinductance with varying current. The operation of the restoring circuit just described may be visualized as follows. At the end of each sweep the current in tube 92 drops abruptly to its quiescent value. The combined currents in coils 91 and 95 and in the damping resistors 911 will now try to change in such a manner as to reach an equilibrium value which is a function of this quiescent current. The change of current in the coils 91, 95' occurs, of course, in accordance with the elfective time constants in the circuit. Furthermore, in the transient state after the tube current has returned to its quiescent value, the changing currents in the coils 9'1, 95 are in opposition. By adjustment of the value of resistor 96 the discharge time constant for coil 95 can be established at a value such that the current therefrom exactly cancels the current in coil 91 at a fixed instant of time. Since each succeeding sweep starts at a fixed time after the end of the preceding sweep, the recovery time is constant and with proper adjustment of .resistor 96 each'sweep starts with a zero valued A.C.

component of expansion force. Fluctuations in the am plit-nde of the vertical expansion currents apply as equal 1 factors to both coils 91, 95 and the cancellation condition obtains irrespective of the magnitude thereof so long as the circuit remains substantially linear.

The deflection system of the present invention incorporates, as shown in FIG. 6, horizontal and vertical deflection arrangements having the features described with refer ence to FIG. 5, the components related to the horizontal deflection arrangement being identified by primed reference characters corresponding to the reference characters identifying the corresponding elements of the vertical arrangement. The variable source indicated as 94 in FIG. is here amplified into a single voltage source 97 with vertical and horizontal regulator circuits 94 and 94', respectively.

present invention overcomes the difiiculties of this nature 1 heretofore encountered and provides displays which may .by S.

1% The Cursor and Wave-0J5 Line Generators The electronic lines on the respective displays are generated with respect to the scanning positions of Az antenna 1111 and El antenna 102 as shown in FIGS. 3, 3A

and 7. These antennas scan limited sectors of space ina manner well known in GCA systems as described, for example, in U.S. Patent No. 2,555,101 to Alvarez et al. Referring to PEG. 7, the antennas are driven in scanning motion by a motor means 103 which drives the two antennas 191, 162 to scan their respective sectors in synchronism such that they each reach the end of the scan motion simultaneously. A signal representative of antenna scan position is obtained by suitable means, as Az angle voltage potentiometer 164 connected to Az antenna 1111 and El angle voltage potentiometer 105 connected to El antenna 1M. The motion transmitted to potentiometers 1114, 1115- is transmitted by mechanical phase cation'Serial No. 247,046, filed September 18, 1951, now

U.S. Patent 2,718,591 to W. R. Hedeman, Jr. The voltage output of the function generators 116-421 is applied to control delay pulse generators 122, 123, 124, 1 25, 126, 127 which supply accurately timed intensification pulses to "the cathode-ray indicator via a video output circuit 128 of FIG. 7a, to which reference will later be made. The theory and operation of the function generator and pulse delay circuits will be explained presently. The intensification pulses when applied to the video amplifier are mixed with the video input and the range mark signals, and the resulting composite signal is applied to the control grid 584 of the CR tube. The aggregate of the intensification pulsesproduces the cursor and wave-off line indications on the display.

Before an explanation is given of the circuit used to develop the electronic cursor and wave-off lines it is desirable to understand the manner in which these lines may be made to simulate straight lines in space. There The location of the trailer is indicated at 84. It is separated from the center line 85 of the runway by -a distance d. The line 8 6 represents the center line of the scan pattern of the Az antenna and the lines 8 7 and 88 represent the limits of scan. The lines 89 and 97 indicate intermediate positions of scan. The distance in space from the antenna to the intersection of the ninway center line and the line of antenna scan is represented It is desired to generate a display on which the line 85 will be formed by dots of beam intensification with one dot being formed during each sweep. On the display the point of origin will correspond to point 84 and the time delay between the start of each sweep and the intensi-fication impulse of that sweep must be proportion to the distance S for the scan position then existing.

Thus the AZ cursor potentiometer (FIG. 7) must have its resistance so proportioned that a voltage proportional to the movement of its slider, as generated by the inverse function generator 118, will, when applied to the control delay pulse generator 122, generate control pulses delayed by such times that a straight line corresponding to 85, made up of dots of intensification, will result.

v the potentiometer 110 can be developed from the trigonometric relationships of the diagram of FIG. 8. The angle [3 represents the angle formed by the zero degrees scan line 86 and the cursor line or runway center line 85. The angle 9 represents the angle formed by the intersection of the scan line 86 and the line of scan at any given instant, such as the line 8%. The angle or represents the angle formed by the intersection of the line of scan at any :given instant, such as 89, and the projected runway center line 85.

The dimension S and the angle or vary as a function of antenna scan for any fixed position of the GCA trailer. It can be seen that a right triangle is formed by the side d, the side S and the segment or" the runway center line intersected by sides d and S. The instantaneous value of S establishes the radar range at which the cursor line spot should appear on the display for each sweep and the side 8 corresponds to the sweep trace. 7

The radar range S from the origin to the point P can be expressed by the equation The sin functions of (5+0) can be eliminated by substituting the quantity (,B-H) expressed in radians, without objectionable error, since the angles are small.

Since in radar, range is a linear function of'time, an equation can be developed which will express S in terms of time or time delay between the transmitted pulse and thereturn echo. This is the equation S=AT where A is a constant equal to 492 feet per micro-second. By substitution the equation is derived. This value of time delay determines where in the azimuth display each intensity modulated cursor spot should appear. it is the function of the delay pulse generators 122 to 127 to supply time delays for the various cursor and wave-off lines and the delay pulse generator 122 performs this function for the Az cursor. The time delay generated by these circuits varies linearly with the input voltage applied thereto by the inverse function generators 116 to 121.

The time delaygenerated by any of the delay pulse generators may be expressed by the'equa-tion T= VDH where D is a constant representing the slope of the linear time delay circuit curve expressed in micro-seconds per volt, with H equal tol. The constant H is a control, functioning as a multiplying factor of D. Solving for V, the voltage applied to the time delay circuit, the equation is derived. This equation may be substituted into the 5 equation 12 to obtain the equation d V- AH 6?) This is the voltage required by the time delay circuit 122 to produce cursor generating pulses;

A diagram of trigonometric relationships similar to that of FIG. 8 may be drawn up tor right of runway operation and equations similar to those above may be derived by the same methods. An equation DAH( s0) will be obtained for the voltage to be supplied to the time delay circuit 122 underthese circumstances.

For determining thevoltage to be applied to the El cursor time delay circuit 127 diagram of trigonometric relationships similar to FIG. 8 may be drawn and equations derived in the same manner. 'It will be found that the voltage may be expressed by the equation d sin 6 DAH( B-0 where d is now the distance on the ground from the trailer to the touchdown point and ,6 is the angle between the glide path and the ground plane.

The method of determining the voltage to be applied to the time delay circuits for the wave-oft lines of the Az display is similar to that followed above. For left of runway operation the No. 1 wave-ofi lineslopes to the left of the runway at a rate of, for example, 200 feet per mile. The equation developed for the Az cursor line maybe used, the only difference being a smaller value of d, land-to achieve the required slope, 3 is replaced by (ti- 119 degrees). This line would be called the No. 2 wave-ofi line in right of runway operation.

The No. 2 Az wave-oil line in left of runway operation i will utilize the equation d sin 5 DAHw-o developed for the El cursor line, can be applied to the El No. l wave-off line, the only diiference being a smaller value of (d). That is,'this wave-ofi line intersects the ground line at a point closer to the trailer than does the El cursor line. V e V The No.2 wave-off line is a straight line in space, intersecting the ground line at a distance -d :from the trailer, intersecting the El curs-or line at a point over the end of the runway, and sloping down from the El cursor line at a rate of approximately fifty f eet per mile. To achieve this slope the No. 2 wave-off line must intersect the ground line at an angle of 3-.48 degrees. The equ-ation . TDAHm-e) developed for the El cursor linemay be applied to the El No. 2 wave-off line, the only difference being smaller values of both d and {3.

The voltages V referred to above are developed by the inverse function generators 116 to 121 (FIG. 7) in response to the positioning of the potentiometers 108 to 13 1 13. The potentiomete'rs are'linear and are driven from the respective antennas through gearingwhich provides approximately 240 of potentiometer slider travel for each complete scan movement of the antenna.

FIG. 9 is a graph showing a curve of resistance of one of the potentiometers, which may be taken to be the Az cursor potentiometer 110 for left of runway operation. It also shows a curve of voltage output of the resistor. FIG. 9a is a simplified schematic of the same potentiometer 110 having end terminals 129 and 130 and a slider 131. The resistance between the end terminals is indicated by R and that between the slider and terminal 129 as r. The output voltage of the poten tiometer is indicated by V 1 FIG. 9 is a graph of r over the range of potentiometer rotation which is indicated as As can be seen from the drawing, the resistance r remains constant as the potentiometer rotates from 2.5 to 49, then increases linearly as rotation continues from 49 to 207. From 207 of rotation to 357.5 the resistance r remains constant at 30,000 ohms. To produce such a curve, the resistance card of the potentiometer is shorted out over the ranges 2.549 and 2073S7.5. The points 49 and 207 mark the limits of cursor line development.

It can be seen that r varies linearly with since (p is a linear function of 6. This relationship can be expressed as r=F0+C. The constant F represents the slope of the resistance antenna scan angle curve of the cursor potentiometer. The constant C represents the value of r when theantenna scan angle is equal to zero degrees. To solve-for the constants F and C substitution may be employed. Considering the range of antenna scan between potentiometer positions at 49 and 207 to be 17.3 we can substitutefor point X, 1

Solving for V in this equation, the equation is developed, Substitutingth e value of rexpressed as a function of 6 and (in degrees) the equation eR i V t 17.3 17.3) is developed. This equation reduces to where {3 and 0 are expressed in radians. I equation of the output voltage of the potentiometer as a function of 6 and ,8. Thepreviously developed equation 1 represented the voltage required by the time delay circuit .to produce therequired cursor generating pulses.

If it is assumed that V =V the two equations can'be cornbined and solved for d. By so doing the equation d=.302 HDAe is produced. 1

It can be seen that, in order to make dconstant for This is the any given situation, the necessary circuit requirement will be that e, the voltage on the area of the poeentiometer, be held constant. This is accomplished by the inverse function generators in a manner which will be explained.

In order to shift from left of runway to right of runway operation the same cursor potentiometer is used but the direction of its rotation is reversed with reference to antenna motion. Also, since it is assumed that the 0 scanline always crosses the runway in front of the antenna and that the antenna scan angle was 5 degrees when the cursor terminated (at theoretically infinite range) in left of runway operation, and is 4-3 degrees for right of potentiometer must Wave-off lines areof similar construction to those related.

to the Az display. In all cases the voltage e on the potentiometer arm must be held constant as the slider moves.

FIG. 7 shows a schematic circuit diagram of one of the inverse function generators, the one selected being the generator 121, associated with the El cursor potentiometer 113. The configuration of this potentiometer is similar to that shown in FIG. 9a, the same terminals 129, and 131 being present.

The circuit comprises a pentode 132 which acts as an amplifier. Voltage from a 150 volt source is applied to; the screen grid by way of a voltage divider composed of resistors 133 and 134 and to the slider of a potentiometer 135 in the cathode circuit through a resistor 136. The anode is directly connected to the control grid of a cathode follower 137, the cathode of which is connected to the negative terminal of the 150 volt source through a pair of resistors 138, 13?. The junction joint of these resistors is connected to the control grid of a second cathode follower 140, the-cathode of which is returned to the negative terminal of the 150 volt source through a resistor 141. The terminal 129 ofthe potentiometer 113 is connected to the cathode of tube and the slider 131 is connected to the grid of tube 132.

The output voltage V of the circuit is taken from the terminal 129 and applied to the circuit 127.

In the operationof this circuit it is desired that the output voltage V vary inversely with the angle 9, and

as discussed above, that the voltage-e ont'he slider 131 be held constant throughout the portion of the excursion of -6 for which a cursor is required. The circuit shown will maintain a substantially constant value of e, provided the gain thereof is infinite.

Assumethat thevalue of e istwo volts positive. As the slider moves toward the terminal 130, in order to maintain a constant value of e, the voltage across the I potentiometer must increase. For example, if the slider is at the terminal 129, and the voltage from. theslider to ground is two volts, the voltage across the potentiometer must be two volts. If the slider is at the mid-point of the resistance winding, and if its value is 30,000 ohms, in

order for e to remain constant at two volts, the.voltageacross the potentiometer must have risen to four volts. When the arm of the potentiometer reaches 22,500 ohms the voltage across the potentiometer must have risen .to eight volts.

Any tendency for the voltage on the arm of the potentiometerto decrease will cause the output voltage of the amplifier 132 to rise rapidly. This voltage rise will be reflected by the two cathode followers 137,141) and will result in a rising voltage at terminal 129. of the poten- 

1. MEANS FOR GENERATING ON THE SCREEN OF A CATHODE RAY TUBE A DISPLAY REPRESENTATIVE OF A REGION IN SPACE SCANNED BY A BEAM OF ENERGY EMITTED BY A RADIO PULSE ECHO SYSTEM, INCLUDING A REPRESENTATION OF A LINE LYING WITHIN SAID REGION OF SPACE, SAID MEANS COMPRISING: MEANS DERIVING FROM THE SCAN OF SAID BEAM AN ANGLE VOLTAGE THE INSTANTANEOUS MAGNITUDE OF WHICH IS REPRESENTATIVE OF THE ORIENTATION OF SAID BEAM WITHIN ITS SCANNING PATTERN, MEANS CAUSING THE CATHODE BEAM OF SAID TUBE TO SWEEP REPETITIVELY ACROSS SAID SCREEN IN SYNCHRONISM WITH THE TRANSMISSION OF THE PULSES OF SAID SYSTEM, SAID SWEEPS HAVING AN ORIENTATION WITH RESPECT TO A REFERENCE POINT ON SAID SCREEN WHICH IS REPRESENTATIVE OF THE SCAN OF SAID BEAM, MEANS DERIVING FROM THE SCAN OF SAID BEAM A SECOND ANGLE VOLTAGE WHOSE AMPLITUDE VARIES INVERSELY AS THE SCAN ANGLE FROM A REFERENCE POSITION, SAID VOLTAGE BEING PROPORTIONED TO THE DISTANCE TO SAID LINE IN SPACE, MEANS GENERATING A TRAIN OF PULSES WHOSE TIME DELAYS FROM RESPECTIVE OUTPUT PULSES OF ENERGY EMITTED BY SAID PULSE ECHO SYSTEM VARY DIRECTLY WITH THE AMPLITUDE OF SAID SECOND ANGLE VOLTAGE MEANS APPLYING SAID TRAIN OF PULSES TO INTENSITY MODULATE THE CATHODE RAY TUBE AND THEREBY PRODUCE ON SAID DISPLAY A TRUE REPRESENTATION OF SAID LINE LYING WITHIN SAID REGION OF SPACE. 